Conventionally, there is well known an air-cooled engine in which equipment, such as a muffler, a fuel tank and an air cleaner, is arranged at a side of a cylinder head, a cooling air from a cooling fan provided on one side of a cylinder is guided into the cylinder head from the side of the cooling fan, and the cooling air passes through a cooling air passage formed in the cylinder head and is discharged at the opposite side (for example, see the Patent Literature 1), and an air-cooled engine that the cooling air is sent to the outside of the cylinder opposite to the cooling fan so as to cool the cylinder (for example, see the Patent Literature 2).
With regard to the cooling construction of the cylinder head shown in the Patent Literature 1, a first induction port and a second induction port are provided at both sides of an intake port communicated with a duct of an intake manifold in the side surface of the cylinder head at the side of the cooling fan, and cooling air is guided through these induction ports into the cylinder head. Guides are provided at both sides of the intake manifold and guide the cooling air from the cooling fan. The cooling air induced through the first induction port flows in a first cooling air passage, formed substantially straightly to a discharge port provided on the opposite side surface of the cylinder head, and is discharged outward through the discharge port. The cooling air induced through the second induction port flows in a second cooling air passage and a third cooling air passage formed between an intake valve and an exhaust valve arranged at the center of the cylinder head, and then joins the cooling air flowing in the first cooling air passage and is discharged outward through the discharge port.                Patent Literature 1: the Japanese Patent Laid Open Gazette Hei. 1-48387        Patent Literature 2: the Japanese Patent Laid Open Gazette Hei. 8-232654        